In fact, knowledge-wise, there are not many in chess. You only have 10 to 20 basic tactics, no matter how deeply you want to categorize them. But we spend years on tactics before we move on to discuss any opening in depth. Yes, your child knows what double attack is, but can s/he notice it on the board? can s/he utilize it even before it shows up on the board? can s/he plan for it?
We practice these skills again and again in classroom, and in homework. We hone in our searching strategies, hoping we could spot them more reliably. They are all experience, not knowledge.
From my point of view, chess is 10% of knowledge, and 90% of experience. To improve and increase our experience, we have to work on tactics problems, play games, and play tournaments. Any knowledge is useful only when they are used.